1. Field of the Invention
The present invention relates to a signal processing method that uses a maximum likelihood decoding technique and also relates to a method for evaluating a read signal by the maximum likelihood decoding technique.
2. Description of the Related Art
Recently, as the densities of optical disc media have been further increased, the shortest length of recording marks has come even closer to the limit of optical resolutions. As a result, intersymbol interference is occurring increasingly often and the SNR (signal to noise ratio) is decreasing even more significantly. To overcome these problems, it has become more and more common to adopt PRML (partial response maximum likelihood) as a signal processing method.
The PRML method is a combination of the partial response (PR) technique and the maximum likelihood (ML) technique, and is a method for selecting a most likely signal sequence based on a read signal waveform on the supposition that a known intersymbol interference should occur. Thus, it is known that decoding performance improves compared to the conventional level determination method (see, for example, Illustrated Blu-ray Disc Reader, Ohmsha, Ltd. (Non-Patent Document No. 1))
Meanwhile, as the level determination method has been gradually replaced by the PRML method as a mainstream signal processing method, some problems are arising about how to evaluate the read signal. Specifically, jitter has been used as an index for evaluating a read signal on the supposition that the signal processing technique is level determination, and therefore, may sometimes have no correlation with the decoding performance of the PRML method, which requires a different signal processing algorithm than that of the level determination. Thus, new indices that do have correlation with the decoding performance of the PRML method have been proposed (see, for example, Japanese Patent Application Laid-Open Publication No. 2003-141823 (Patent Document No. 1) and Japanese Patent Application Laid-Open Publication No. 2004-213862 (Patent Document No. 2)).
On the other hand, a new index that would contribute to detecting edge shifting between marks and spaces, which are very important factors that determine the storage quality of an optical disc medium, has also been proposed lately (see, for example, Japanese Patent Application Laid-Open Publication No. 2004-335079 (Patent Document No. 3)). As long as the PRML method is adopted, this index should also comply with the PRML rule and should also have correlation with the decoding performance of the PRML method. In addition, this index also needs to represent quantitatively the magnitude and direction of edge shifting on a pattern-by-pattern basis.
As described above, as the densities of optical disc media have been further increased, the intersymbol interference and SNR will have more and more serious consequences. Non-Patent Document No. 1 (Illustrated Blu-ray Disc Reader, Ohmsha, Ltd.) discloses that to maintain a system margin, the PRML methods need to be changed into a higher-order one. For example, if a 12 cm optical disc medium has a storage capacity of 25 GB per recording layer, the system margin can be maintained by adopting PR 1221 ML method. However, if the storage capacity per recording layer is 33.3 GB, then PR 12221 ML method should be adopted. It is expected that as the densities of optical disc media are further rising, the order of the PRML method to adopt should continue to get higher and higher in this manner.
Patent Document No. 1 (Japanese Patent Application Laid-Open Publication No. 2003-141823) and Patent Document No. 2 (Japanese Patent Application Laid-Open Publication No. 2004-213862) disclose that “difference metrics representing the difference between the first most likely state transition sequence and the read signal and the difference between the second most likely state transition sequence and the read signal” should be used as index values. In that case, if the “first and second most likely state transition sequences” that could produce errors have multiple patterns, then those patterns should be processed statistically. However, Patent Documents Nos. 1 and 2 do not disclose such a processing method.
Paying special attention to this point, Patent Document No. 5 (Japanese Patent Application Laid-Open Publication No. 2003-272304) discloses a method for detecting multiple patterns of the difference metrics that have been detected by the same method as the ones disclosed in Patent Documents Nos. 1 and 2 and processing those groups of patterns. According to the PR 12221 ML signal processing method disclosed in Patent Document No. 5, there are three groups of patterns (i.e., groups of patterns of merging paths with a relatively short Euclidean distance) that are likely to produce errors. These groups of patterns have mutually different probabilities and generate respectively different numbers of errors. That is why Patent Document No. 5 discloses a method in which a standard deviation σ is calculated based on the distribution of index values that have been obtained from those patterns and in which the errors to produce are predicted based on the probability of occurrence of the patterns (i.e., the frequency of occurrence with respect to the total parameters) and the number of errors to produce when the patterns are erroneous. Patent Document No. 5 adopts, as a method for predicting the errors, a method in which the distribution of the index values thus obtained is supposed to be a normal distribution and in which the probability of the index value becoming equal to or smaller than zero (i.e., the probability of causing bit errors) is predicted based on the standard deviation σ and the average variance μ thereof. This is a normal technique for predicting the probability of occurrence of errors. According to Patent Document No. 5, the probability of occurrence is calculated on a pattern-by-pattern basis, thereby calculating the expected error rate and using that expected error rate as an index to signal quality.
Patent Document No. 4 (Japanese Patent Application Laid-Open Publication No. 2003-51163) and Patent Document No. 5 mentioned above disclose a method that uses bER, predicted based on the difference metrics, as an index. However, even if those parameters were used as index values, those parameters would have no compatibility with jitters on the time axis that have been used as an index for evaluating the signal quality of an optical disc, and would be difficult to handle. Among other things, to ensure compatibility with the conventional jitter index values, signal evaluation indices that can represent the signal quality with σ are particularly needed. A system that is supposed to use the PR 12221 ML signal processing technique as disclosed in Patent Document No. 5 needs a signal evaluation index that can be represented by σ and that can indicate a signal quality that is highly correlated to the performance (as represented by the error rate) of the PR 12221 ML signal processing. As described above, according to the PR 12221 ML signal processing, there are three different groups of patterns (i.e., groups of patterns of merging paths with a relatively short Euclidean distance) that are likely to cause errors. FIG. 18 shows the distribution of the squares of the difference metrics in those three groups of patterns. Those three groups of patterns have mutually different probabilities and respectively different numbers of errors to occur, and therefore, it is difficult to represent those three groups of patterns as a single distribution.